11 Definitions

The new coordinate systems introduced by the IAU have direct consequences on the definition of concepts and quantities that are used, or should now be used, in fundamental and applied astrometry. As a service to the user, we provide here these definitions:

• Astrometric position: observed positions with respect to GCRF reference stars at some epoch. Positions have not been corrected for annual and planetary aberration.

• Barycentric Celestial Reference System (BCRS): a global "space fixed'' coordinate system centered at the barycenter, defined by the metric tensors of the IAU 2000 resolutions (Soffel 2000).

• Catalog, or mean, positions: positions on the ICRF.

• Celestial Ephemeris Origin (CEO): origin for the right ascensions on the true equator of date. Its motion has no component along the true equator.

• Celestial Intermediate Equator, or the true equator: equatorial plane through the center of the Earth and perpendicular to the celestial intermediary pole (CIP) at some epoch.

• Celestial Intermediate Pole (CIP): equatorial pole defined by the IAU 2000A precession-nutation model which is the transformation from the ICRF to the true equator.

• Earth rotation angle: see Stellar Angle.

• Ecliptic: mean plane of the Earth's orbit around the Sun as determined from and designated by an ephemeris of the Earth, (e.g. ecliptic of DE 405).

• Ecliptic longitudes and latitudes: coordinates measured from the equinox with respect to the ecliptic of date in TCB. They can be apparent geocentric, true, or mean.

• Equinox: either of the two points at which the ecliptic intersects the celestial intermediate equator; also the time at which the Sun passes through either of these intersection points; i.e. when the apparent longitude of the Sun is 0 or 180 degrees. When required the equinox can be designated by the ephemeris of the Earth from which it is obtained (e.g. vernal equinox of DE 405).

• Equinox right ascensions: right ascensions that are measured from the equinox, instead of from the CEO.

• Fixed ecliptic: ecliptic for any ephemeris at epoch J2000.0, which is orientated such that the equinox is aligned with the Ox origin of the ICRS.

• Geocentric apparent right ascensions and declinations: positions measured in the GTRF of specific time. They are topocentric positions that have been corrected for diurnal aberration and geocentric parallax (e.g. the geocentric apparent right ascension and declination at epoch 2002 June 15.312 on TCG or TT).

• Geocentric Celestial Dynamical Reference System: it is used for the representation of the motion of artificial satellites. It differs from the GCRS by subtracting the geodetic precession from the current precession-nutation series. The equations of motion should include the additional terms of acceleration of the axes due to the motion of the Earth.

• Geocentric Celestial Reference System (GCRS): a geocentric "space fixed'' coordinate system, defined by the metric tensors of the IAU 2000 resolutions and related to the BCRS by generalized Lorentz transformations (Soffel 2000). It has no kinematic rotation since it is based on extragalactic sources and its axes are parallel to those of the ICRS.

• Geocentric ICRF positions: defined by the ICRF origin and equator, but are centered at the geocenter.

• Geocentric True Reference Frame (GTRF): geocentric reference frame defined by the true equator, CIP, and CEO at a specified date.

• Geocentric true right ascensions and declinations: coordinates given in GTRF at a specific time in TCG or TT. They are geocentric apparent positions corrected for annual aberration and gravitational deflection (e.g. the true geocentric right ascension and declination at time 2002 June 15.0 in TT).

• Geometric position: the position of an object in a three dimensional reference frame that can be either topocentric, geocentric, or barycentric with the Oxyz axes parallel to the ICRF axes.

• International Celestial Reference Frame (ICRF): radio realization of the barycentric fixed celestial reference system (ICRS). Other realizations have a specific name (e.g. HCRF: Hipparcos Celestial Reference Frame).

• International Celestial Reference System (ICRS): the international reference system is a fixed epoch independent system with the assumption that the extragalactic sources do not show a global rotation. It assumes in addition a relativistic space-time.
• International Terrestrial Reference Frame (ITRF): a realization by a set of coordinates and velocities of fiducial points on the Earth (e.g. ITRF-2000).

• International Terrestrial Reference System (ITRS): the terrestrial reference system to which the positions on the Earth are referred. It is geocentric and has no global residual rotation with respect to horizontal motions at the Earth's surface.

• Local positions: the observed position only corrected for refraction and instrumental parameters.

• Right Ascension $\alpha$ and Declination $\delta$: without qualification, right ascension and declination are measured in ICRF. The epoch of the positions must be specified. By extension, catalog or mean position in a barycentric reference system from the catalog right ascension fiducial point and from the catalog equator, respectively. They are designated with the catalog reference system as necessary (e.g. Hipparcos right ascension and declination at epoch 1991.25 on TCB).

• Stellar Angle, or Earth Rotation Angle: angle measured along the true equator between the TEO and the CEO positively in the retrograde direction.

• Terrestrial Ephemeris Origin (TEO) is the origin of the longitudes in the terrestrial reference system of date.

• Topocentric apparent position: position on the ITRF and TT where a local place has been corrected for diurnal aberration. Alternatively, a geocentric apparent position has been transformed for Earth rotation and polar motion and corrected for diurnal parallax and light bending.